1. Field of the Invention
The present invention relates to photoimageable epoxy compositions useful as photoresists for applications requiring thick films. In particular, this invention relates to specific composition useful for that purpose that contains cyclopentanone as a solvent, at least one epoxidized polyfunctional bisphenol A formaldehyde novolak resin and at least one photoacid generator.
2. Brief Description of Art
Advanced packaging applications requiring solder bumps having a high aspect ratio (i.e., ratio of the film thickness or height to the width of the developed imaged feature) and applications involving the fabrication of micro electromechanical machines (MEMS) require photoresists which are capable of producing uniform layers by spin-coating, providing high aspect ratio images up to thicknesses of several hundred microns. Conventional positive resists based on diazonaphthoquinone-novolak chemistry are not suitable for applications for which the thickness is required to be above about 50-60 microns. This thickness limitation is primarily caused by the relatively high optical absorption of the diazonapthoquinone-type photoactive compound at the near-ultraviolet wavelengths (350-450 nm) which are typically used to expose the resist. Optical absorption necessarily reduces the radiation intensity as it traverses from the top to the bottom of the film, such that if the optical absorption is too high, the bottom of the film will be underexposed relative to the top, causing a tapered or otherwise distorted profile of the developed image.
A negative spin-coating thick-film resist of the chemically amplified type, which has very low optical absorbance at wavelengths in the 350-450 nm range has been described in the literature (N. LaBianca and J. D. Gelorme, Proc. SPIE Vol. 2438, page 846 (1995). This resist comprises a solution in a coating (casting) solvent of a polyfunctional epoxy novolak resin EPON(copyright) SU-8 from Shell Chemical and a triphenyl sulfonium hexafluoroantimonate salt (PAG), such as CYRACURE UVI 6974 from Union Carbide. The EPON SU-8 has a high functionality which results in efficient cross-linking, so the resist is capable of forming images having a high aspect ratio in films which may be 300 microns or more thick. The PAG is added at a concentration of less than 10% of the total solids. The disclosed coating solvent uses a mixture of gamma-butyrolactone (GBL) and methyl ethyl ketone (2-butanone). The resulting photoresist solution may be spin-coated onto a wide variety of substrates, pre-baked to evaporate the solvent, leaving a solid photoresist film of up to several hundred microns thickness depending on the solids content of the solution. A pattern is transferred to the photoresist by exposing the film through a mask to near-ultraviolet radiation by contact, proximity, or projection exposure. Subsequent immersion in developer solution dissolves away the unexposed regions, leaving behind a high resolution three dimensional negative image of the mask.
A further reference (K. Y. Lee, N. LaBianca, S. A. Rishton, J. D. Gelorme, J. Shaw and T. H. P. Chang, J. Vac. Sci. Technology B 13(6), 1995) discloses similar spin-coating photoresist compositions, in which the solvent is the single solvent gamma-butyrolactone.
Previous disclosures of thick-film negative photoresist compositions containing SU-8 resin, and a solvent are as follows: U.S. Pat. No. 4,882,245, assigned to International Business Machines Corp. discloses preferred photocurable compositions suitable for use in a printed circuit board which contain up 88% by weight of SU-8 resin, CYRACURE PAG, a reactive diluent epoxy resin and methyl ethyl ketone or methyl isobutyl ketone (4-methyl-2-pentanone) or mixtures thereof as the solvent. The preferred solvent was methyl ethyl ketone alone. These compositions are applied to a cured epoxy resin substrate by means of a doctor-blade and not by the spin-coating method of the present invention.
U.S. Pat. No. 5,026,624, assigned to International Business Machines Corp. discloses a preferred photocurable compositions suitable for use as a solder mask which contain up 90% by weight of SU-8 resin, CYRACURE PAG, a reactive diluent epoxy resin and a solvent selected from gamma-butyrolactone, N-methyl pyrrolidinone, 2-ethoxyethyl acetate, propylene carbonate, 2-methoxyethanol, propylene glycol monomethyl ether or propylene glycol monomethyl ether acetate. The preferred solvent was propylene glycol monomethyl ether acetate. These compositions are applied to a cured epoxy resin substrate by means of curtain coating and not by the spin-coating method of the present invention.
Negative photoresists based on the above disclosed compositions which are suitable for spin-coating, in which the solvent is gamma-butyrolactone alone, are sold by MicroChem Corp., Newton, Mass., USA and used commercially, especially in the fabrication of MEMS devices. For example, a typical product offered by MicroChem Corp., xe2x80x9cSU8-50xe2x80x9d can be spin-coated at 1000-3000 rpm to produce films of thickness in the range 30-100 microns, which after exposure and development according to a standardized process can produce images having an aspect ratio greater than 10:1 at film-thicknesses greater than 100 microns. Higher or lower solids versions extend the film-thickness range obtainable by a single coat process to below 5 microns and above 200 microns. Gamma-butyrolactone is selected as the solvent for these photoresist compositions primarily because of its high solvency of the SU-8 epoxy bisphenol A resin, and because it is non-flammable. Additionally, the viscosity of a solution containing a high concentration of solids required for a very thick film resist can reduced substantially by heating to a moderate temperature, thus making it practical to be filtered under reasonable pressure to remove particles or gels.
The choice of the coating (casting) solvent is critical. The use of a low boiling point high volatility coating solvent, such as methyl ethyl ketone, which has a vapor pressure of 100 mm at 25xc2x0 C., may cause the photoresist film to dry too quickly resulting in an uneven surface. The mask pattern to be imaged cannot then be transferred uniformly across the area of the film. If the film is not dried sufficiently, the surface will remain tacky, in which case the film will stick to the mask if contact printing is used to transfer the pattern. Additionally, a stable image may not be formed. It is thus generally preferable to use a relatively high boiling point low volatility coating solvent with a vapor pressure of less than about 10 mm for photo resists which are to be deposited as uniform films by spin-coating.
However, the drying time, which increases rapidly with film-thickness, may become rather long if a low volatility solvent is used for a thick-film forming resist composition. This makes the resist unsuitable for use in production processes in which high throughput is important. In the case of the above disclosed compositions using as the solvent gamma-butyrolactone which has an extremely low vapor pressure of 0.3 mm, the drying time at practical bake temperatures (typically below about 100xc2x0 C.) is in the range of 10-90 minutes for film-thicknesses in the 20100 micron range. Furthermore, the coating uniformity tends to be poor towards the outer edge of the substrate due to the formation of a rather large edge-bead, and the resist solution may not wet the surface of the substrate sufficiently resulting in uneven spreading, de-wetting or pulling-back of the film from the edge of the substrate after spin-coating or after soft bake. This occurs particularly on substrates such as bare silicon, silicon nitride, glass, or certain metals such as copper. Although the wettability of the substrate may be improved, for example, by treating bare silicon by means of sulfuric acid/hydrogen peroxide followed by hydrofluoric acid etching (piranha etch), or by using a selected primer, this adds to the complexity of the coating process, and in the case of certain primers can lead to additional problems such as lifting-off of imaged features during development.
Reducing the drying time by mixing a high volatility solvent with gamma-butyrolactone is possible; however, this is undesirable because the composition of the solvent mixture can change during mixing with the solid resist components due to loss by evaporation of the highly volatile solvent, which makes the manufacture unnecessarily complex. Furthermore, the solubility may decrease and a higher solids content may be required to achieve the same film thickness as would be obtained using gamma-butyrolactone alone. Furthermore, solvents of high volatility have flash points below room temperature. Such mixtures in which methyl ethyl ketone is the volatile solvent are not used commercially because methyl ethyl ketone is a known toxic substance.
Therefore, the present invention is directed to a composition useful for a thick-film negative resist comprising a mixture of at least one epoxidized polyfunctional bisphenol A formaldehyde novolak resin, at least one photoacid generator, and wherein cyclopentanone comprises at least the majority amount (i.e., at least 50% by weight) of the casting solvent. The resulting mixtures having a wide range of total solids content, may be applied by spin-coating in a single coating step to a variety of substrates, and baked to produce tack-free non-brittle films having a range of thickness up to several hundred microns.
An advantage of the present invention is the fast drying time, which is afforded with uniform coatings and with no increase in cracking of the film during the drying process or during the post-exposure bake.
Another advantage is the improved wetting obtained on different substrates such as bare silicon, glass, silicon nitride and other surface coatings commonly used for MEMS applications.
Yet another advantage is that cyclopentanone may be used without a co-solvent.
Yet another advantage is that only a small increase in the solids concentration is required to achieve the-same film thickness as would be obtained using gamma-butyrolactone. Furthermore, cyclopentanone solvent is not known to be toxicologically hazardous and has a flash point above room temperature.
As stated above, the present invention is directed to a composition comprising:
(a) at least one particular polyfunctional epoxy resin which is capable of undergoing ring-opening polymerization under the influence of a strong acid catalyst.
(b) at least one particular photoacid generator which releases a strong acid under the influence of ultraviolet light,
(c) cyclopentanone as the major component of a solvent which is capable of dissolving all ingredients. Optionally, a surfactant may be added to further improve coating uniformity.
The term xe2x80x9cepoxidized polyfunctionalxe2x80x9d as used herein to describe bisphenol A formaldehyde novolak resins includes any and all resins of this class that contain sufficient epoxy functionalities to be useful as a thick-film negative photoresist. The preferred epoxy resin contains an average of eight epoxy groups, consisting of the glycidyl ether of the novolak condensation product of bisphenol A and formaldehyde, with an average molecular weight of about 1400 gram/mole, with an epoxy equivalent weight of about 215 gram/mole. Such a resin is commercially available from Shell Chemical under the trade name EPON(copyright) Resin SU-8. The structure of the most common component of the resin having an epoxy functionality of eight is shown in Formula (1) 
The preferred photoacid generator consists of a triaryl sulfonium salt with structure shown in Formula (2), where Ar represents a mixture of aryl groups. Such a material is commercially available from Union Carbide under the trade name CYRACURE(copyright) Photoacid Generator UVI-6974, which consists of a 50 % solution of the triaryl sulfonium salt of Formula (2) dissolved in propylene carbonate. 
Cyclopentanone may be used as a single solvent or as the major component being not less than 50% in a mixture with a co-solvent capable of dissolving the solid components. Suitable co-solvents are polar solvents such as dimethylformamide and N-methyl pyrrolidinone, ketonic solvents such as, cyclohexanone, heptanone, methylamyl ketone and methyl isopropyl ketone, cyclic ethers such as 1,3-dioxolane and tetrahydofuran, hydroxylic polar solvents such as tetrahydrofurfuryl alcohol, ethyl lactate, propyleneglycol methylether, propyleneglycol methylether acetate, methylmethoxy propionate, 2-ethoxyethyl acetate, propylene carbonate, 2-methoxyethanol, ethylethoxypropionate and the like. More preferably, the coating comprises either all cyclopentanone or about 70%-95% cyclopentanone and about 5-30% by weight of one or more of those cosolvents.
Other additives, well known to those skilled in the art, which may be optionally used to improve the coating quality of a spin-coating resist formulation include leveling agents, wetting agents and adhesion promoters, such a nonionic surfactants may be preferably added at about 0.02 to 0.1% by weight. One suitable nonionic surfactant is FC430 from 3M.
A solution of the components suitable for spin-coating is formed by dissolving the polyfunctional epoxy bis-phenol A novolak resin, the photoacid generator and other additives if used in the selected coating solvent system comprising as the major component cyclopentanone. The resin is typically dissolved in the solvent system to a concentration of about 30-to 80 wt %. The photoacid generator (as a solid), is typically present in an amount of about 2 to 6 parts per hundred of the resin, or as a solution in propylene carbonate in an amount of about 4 to 12 parts per hundred. The solution is filtered through a polypropylene or Teflon filter to remove particulate matter. The pore size varies from about 2 to about 10 microns depending on the total solids content of the solution. The photoresist coating is formed by spin-coating the solution onto a substrate to form a uniform thick-film of thickness of at least about 20 microns, preferably 20-300 microns.
The types of substrates can be, but are not limited to, silicon, glass or ceramic which may have deposited on their surface other materials such as silicon oxide, silicon nitride, various metals such as gold, copper or nickel, or alloys.
No special pre-treatment is necessary; optionally, a pre-bake may be performed to remove absorbed moisture. The resulting photoresist solution may be applied to a substrate by spin-coating, consisting of dispensing the liquid resist onto the substrate, accelerating to a constant rotational speed, and holding the rotation speed constant to achieve the desired coating thickness. Spin-coating may be performed with variable rotational velocity in order to control the thickness of the final coating. After spin-coating, a bake is performed to evaporate the solvent; typically 1 minute at 65SC followed by 5-15 minutes at 95xc2x0 C. depending on the film-thickness. The solid photoresist coating can then be photoimaged using an exposure tool with near-ultraviolet radiation from a medium- or high-pressure mercury lamp through a photomask containing a pattern of opaque and transparent regions. Contact, proximity, or projection printing may be used, preferably contact printing. Following exposure, a post-exposure-bake is carried out in order to accelerate the acid catalyzed polymerization reaction in the exposed regions of the coating; typical bakes are carried out for 1 minute at 65xc2x0 C. and 5-20 minutes at 95xc2x0 C. depending on the film-thickness. The coating is then developed in an organic solvent developer in order to dissolve away the un-polymerized regions, typically for 2-10 minutes depending on the film-thickness. Suitable developer solvents include, but are not limited to, propylene glycol methyl ether acetate, gamma-butyrolactone, diacetone alcohol and ethyl lactate. Optionally, a post-bake may be performed on the resulting image to more fully harden the material.
The resulting three dimensional structures obtained in the described fashion may be used for electroplating molds, microelectromechanical systems (MEMS) devices, microfluidic devices, and other applications apparent to those skilled in the art.